Archaeologist Robin Beck has spent decades puzzling over the missing ruins of North Carolina’s Fort San Juan. Historical records show Spanish explorer Juan Pardo established the fort in 1566 to help build a road to silver mines in Mexico.

Yet in excavations, Beck could find no telltale evidence of bastions or palisades — just the remains of five Spanish houses, inexplicably close to a Native American mound.

So Beck and colleagues trained magnetometers on the earthen mound, which revealed a startling contrast between the magnetic properties of the soil and unusual structural frames beneath it. By June, the archaeologists realized they’d stumbled on their buried treasure: the makings of a classic European fort, complete with a moat.

The 16th-century fortifications survived only a year and a half before a neighboring Mississippian tribe burned the Spanish settlement and killed all but one of the soldiers.

Had the fort lasted a little longer, or the soldiers stumbled upon the gold-laden creeks nearby, Beck conjectures, everything south of the Mason-Dixon Line could have been claimed by Spain: “The Spanish crown would have basically brought all the forces of colonialism right down on top of the Carolina Piedmont.”

[This article originally appeared in print as "Lost Spanish Fort Finally Revealed."]